Fix parsing of pseudo-elements and pseudo-classes after ::details-content

[rorshopping]

Fixes #1244

Problem

Selectors chaining pseudo-elements off ::details-content\ fail to parse:

\\css
::details-content::before { color: red } /* error: Invalid state */
details::details-content::after { content: '' }
\\

Root cause

::details-content\ is an element-backed pseudo-element: it behaves like a type selector, so other pseudo-elements and pseudo-classes may follow it. The parser did not model this distinction, and treated every known pseudo-element the same way as ::before\ (which cannot be followed by further pseudo-elements).

Changes

• Add \is_element_backed()\ to the \PseudoElement\ trait (default \alse), implemented for ::details-content\ in lightningcss.
• Track a new \AFTER_ELEMENT_BACKED_PSEUDO_ELEMENT\ parsing state; element-backed pseudo-elements continue to reject type selectors, classes, ids and combinators after them, but allow further pseudo-elements and pseudo-classes.
• Allow functional pseudo-classes (e.g. :lang(), :dir()) after ::part()\ and element-backed pseudo-elements, matching upstream selectors (Firefox bug 1960561) and the spec.

Now all of these parse:

\\css
::details-content::before { color: red }
::details-content::after { content: '' }
::details-content::marker { color: red }
details::details-content::before { color: red }
details::details-content:lang(en)::before { content: '' }
::part(foo):lang(en) { color: red }
\\

while these remain errors (matching browsers):

\\css
::details-content div { color: red } /* error /
::details-content.foo { color: red } / error /
::details-content::part(x) { color: red } / error /
::details-content::slotted(div) { color: red } / error /
::details-content::before::after { color: red } / error */
\\

Tests

• selectors: \ est_pseudo_on_element_backed_pseudo\ (iterating ::details-content::before), mirroring the upstream selectors test.
• lightningcss: minify tests for ::details-content::before/after/marker\ and \details::details-content::before, plus an error test for ::details-content::part(foo).

Note: the selectors crate has a pre-existing \ est_parsing\ failure (the test asserts ::target-text\ / ::search-text\ parse, but \DummyParser\ does not implement those pseudo-elements); it is unrelated to this change.